Explore privacy image generation performance improvements

[jbmorley]

When I was working on #590, I came across an NSHipster article which suggested that image scaling performance varied vastly between different options and that the CIImage approach might be one of the slower ones. It might be worth revisiting this to see if we can make updates more responsive.

https://nshipster.com/image-resizing/

Of course, perhaps the easiest approach would be to generate the privacy mode once (as @tomsci had in his original implementation), rather than generating it on every upload (though this does gain us some flexibility).